The method described below relates to the field of packet data transmission inside mobile radio networks, and more precisely to a method for safely UL/DL transmitting short ACK/NACK bitmaps in ARQ process inside EDGE compliant systems (used acronyms are given at the end of the description).
ARQ signalling by so called “acknowledge” (ACK) and “not acknowledge” (NACK) signals is usual in telecommunication protocols. Such kind of ACK/NACK signalling is described, for example, in 3GPP TS 44.060 V7.3.0 (ex GERAN 04.60). Within EDGE there are nine MCSs, known as: MCS1, . . . , MCS9.
In FIG. 1 a first type of GERAN data block has a first field as “header” and a second field as “payload”, The header includes both BSN and TFI fields. The latter is also designated as “RLC block”. MCS 1 to MCS6 coded blocks are characterized by these two fields. For MCS7 to MCS9 coded blocks three fields are used, a first field for the header, a second field for payload, known as “block RLC1”, and a third field also for payload, known as “block RLC2”. The header includes the TFI, a first BSN1 field assigned to the block RLC1, and a second BSN2 field assigned to the block RLC2.
The RLC ARQ functions available in GERAN support three modes of operation: RLC acknowledged mode, RLC unacknowledged mode and RLC non-persistent mode. RLC acknowledged mode operation uses retransmission of RLC data blocks to achieve high reliability. RLC unacknowledged mode operation does not utilize retransmission of RLC data blocks. RLC non-persistent mode operation uses non-exhaustive retransmission of RLC data blocks.
Known ARQ with ACK/NACK feedback messages, such as PDAN or PUAN, reports bitmaps which occupy a whole radio block. Whenever there is a polling request transmitted in downlink (DL) direction by the base station for receiving back a PDAN message transmitted by the mobile in uplink (UL) direction to either acknowledge or not acknowledge RLC blocks received in downlink, uplink resources are used only for sending the uplink signalling message PDAN. These uplink resources therefore can not be used for sending data. Dual conclusions for waste of downlink resources should be drawn for a PUAN message issued by the base station in downlink to either acknowledge or not acknowledge RLC blocks transmitted by the MS in uplink. In consequence of above, the data transmission might be severely hurt by frequently transmitted ACK/NACK messages, especially in case of transmission over few timeslots.
EP-A-1465371 discloses a flexible polling method that is used by the network to request an ACK/NACK bitmap from a UE. The RLC/MAC header of a block is coded separately from the data.
European patent application No. 05023668.6, filed on Oct. 28, 2005 in the name of the same Applicant discloses a “method to ACK/NACK signalization” which has to be considered under the Article 54(3) EPC. According to the relevant citation:                A first unit (BS) transmits a polling request at a point of time to the second unit (MS) and the polling request initializes an ACK/NACK examination of received RLC blocks there. The second unit examines received RLC blocks, which are assigned to a dedicated timeslot number of the carrier, and the examination is done for all timeslots of the set.        During the examination one binary bit is used to indicate, if a considered RLC block shows errors or not. The indicating bits are used to form a short bitmap and the short bitmap is transmitted from the second unit to the first unit as ACK/NACK signal.        The first unit analyses the short bitmap and identifies erroneous RLC blocks with respect to the transmission timing of the RLC blocks between the first and the second unit, with respect to the assigned set of timeslots and with respect to the point of time the polling request was sent, each of that facts being known at the first unit.        In case of data transmission in uplink direction, the short bitmap will be part of a “normal” uplink data block. Because of this, there is the possibility to add and transmit normal payload within the data block, altogether with the short bitmap block. This feature is present in the uplink RLC block of FIG. 2.        If there is no data transmission in uplink direction the short bitmap will be sent in uplink direction in a so called “access burst”. The transmission and the reception of that kind of burst is normally done without disturbance, the transmission of the short bitmap is therefore save. If the access burst carries the short bitmap then battery power at the mobile station can be saved. Because of the slower transmission repetition of the access burst it is possible to reduce interference within the system/GERAN system.        
Under the cited method the transmission of block sequence numbers as part of the ACK/NACK signal is not needed. Instead of a block sequence number based ACK/NACK reporting, a time based reporting is used. The base station, and hence the mobile station, knows the transmission time and the timing of a dedicated radio block exactly, so the assignment of a received ACK/NACK indication to a formerly sent radio block is possible. For example, if a polling indication is received at a frame number N, the MS would send back a short bitmap for ACK/NACK, indicating the status of all received radio blocks in assigned timeslots during frame number N, N−1, and so on. This is depending on the size of the short bitmap and of the number of assigned timeslots. In most cases a very short bitmap is sufficient for ACK/NACK signalling. If we assume a mobile station with 4 assigned timeslots in downlink direction and a polling period of 40 ms, there will be a maximum of 4×2=8 radio blocks carrying a maximum of 16 RLC blocks submitted during two successive polling.
Since there might be two RLC data blocks per radio block (in case of MCS 7, 8, 9), at most two bits per radio block are needed in the bitmap. For every radio block received in the assigned timeslots, the receiver shall set the pair of bits in the short bitmap as described by the coding rule table of the citation shown in FIG. 3.
In case of multiple TBFs allocated to the same mobile, the bitmap could encompass the information for all TBFs. In this case bits would be set to 1 for RLC blocks correctly received with any of the assigned DL TFIs. This would further optimize the procedure since feedback for all the TBFs could be provided at the same time. FIG. 4 shows how the coding rules of FIG. 3 apply for the generation of a short bitmap. In the figure a DL TBF is allocated on timeslots 0, 1, 2 & 3 (TBF1) and time multiplexed with other TBFs (TBF2 and TBF3) allocated on the same carrier as TBF1. The length of the short bitmap (relevant to the only TBF1) is assumed to be of 2 octets (sequentially read) each one spanning the 20 ms period of a radio block. As the polling is received by the MS in frame N, the first pair of circled bits in the short bitmap shall refer to the radio block received on the first assigned timeslot of frame N, the second pair of bits shall refer to the radio block received on the second assigned timeslot of frame N, etc. Because there is still free space in the bitmap, the next pair of bit shall refer to the radio block received on the first assigned timeslot of frame N−1 and so on.
The method of the cited application can be used for ACK/NACK reporting both for uplink or downlink transmissions, indifferently, although PDAN message is the only one described described below. The instant of the PDAN transmission is scheduled by the base station with a polling request (RRBP) issued in the header of a RLC block transmitted in downlink. There are no reasons for a mobile to poll the network (base station) for transmitting a feedback for its own uplink transmissions, because the network is the master of the scheduling on the downlink channel. Starting from the fast ACK/NACK uplink reporting for downlink transmissions, some other questions have to be considered in order to adapt the faster MAC protocol to the uplink data transmissions. First of all, a criterion must be implemented to inform the mobile that a short bitmap is used instead of traditional extended one with a PUAN message.
The method of the cited application indicates a criterion of coexistence between extended and short bitmaps valid for the only ACK/NACK reporting set in uplink direction. The criterion avails of a substantial redefinition of both known RRBP and ES/P fields in the headers of the EGPRS downlink data blocks. Besides, in order for the base station receiver to know whether or not a short ACK/NACK bitmap is piggybacked in the RLC uplink data block, a spare bit in the UL RLC/MAC header is used. A spare bit exists in all three EGPRS UL header types and will be used for this case.
The gap for downlink reporting is filled up by the temporary 3GPP document TSG GERAN#29, Tdoc GP-060755, San Jose Del Cabo, Mexico, 24-28 Apr. 2006 which suggests:
ACK/NACK reporting set in uplink direction:
                The spare bit in the UL RLC/MAC header is used. Reporting from the mobile of either short or extended bitmap is signalled by the network by using the poll (RRBP) and USF fields in the downlink direction.        
ACK/NACK reporting set in downlink direction:                The spare bit in the DL RLC/MAC header is used. The same criterion of the cited application for ACK/NACK reporting in uplink direction is used for downlink reporting. To say, both RRBP and ES/P fields in the header of the EGPRS downlink data blocks are redefined for this aim.        
The ACK/NACK reporting as it results from the combined teaching of both the priority documents is nevertheless not optimal for the following reasons:                A residual possibility of error exists even decoding the short bitmap with separate CRC. This mainly depends on the MCS adaptively selected for payload and hence for the included short bitmap.        A too rigid mechanism for ACK/NACK reporting in downlink direction which considers reporting for a single TBF only, without considering the eventuality of additional reports from other MSs sharing the same timeslots inside the predetermined reporting window.        